Ettringite is a sulfate double salt with calcium aluminate and has the formula C.sub.3 A.3CaSO.sub.4.32H.sub.2 O (throughout the specification, the following cement chemist notation will be used in accordance with the general practice in the field of cement chemistry: C.dbd.CaO, A.dbd.A.sub.2 O.sub.3, S.dbd.SO.sub.3, and H.dbd.H.sub.2 O). It forms in the initial stage of hydration of portland cement, and once was thought to be a substance harmful to concrete. Later, however, the study by G. L. Kalousek et al., (G. L. Kalousek: "Sulfoaluminates of Calcium as Stable and Metastable Phases," Ph. D. Thesis, University of Maryland (1941)) revealed that the crystals of ettringite, when formed in an unsaturated solution of Ca(OH).sub.2, are prisms or needles, which show great crystal pressure. Since then, it was suggested that ettringite could be used for reducing dry-shrinkage of concrete and for introducing prestress into concrete and, under the stimulus thereof, active development studies have been made to utilize the expansion effect of ettringite, the starting point being used in expansive cement.
Thus, ettringite is now known to be useful as an expansion-producing admixture for cement. Its use is roughly divided into two categoires. One is shrinkage compensating concrete for reducing drying shrinkage, and the other is chemically prestressed concrete for introducing prestress into a structure by taking advantage of the expansive power of ettringite. The former concrete is mainly intended for preventing cracking and is used in slugs for making water tanks, reservoirs, dams, buildings, pavement, bridges and so on, while the latter is mostly intended for incresing cracking resistance or cracking load of factory products, such as concrete pipes, precast concrete floor boards, concrete sheet piles, concrete box culverts and composite concrete-steel pipes.
It is important in an expanded admixture usable for these various purposes that, in hydration, ettringite or its monosulfate, does not precipitate in the liquid phase but that it be formed on the surface of the solid grain, which is the support for hydration. It is clear, as shown in FIG. 1 of the attached drawings, that ettringite crystals should grow on the surface of solid grains, whereas the form of the solid grains remains the same after as before hydration. It has been thought that this ettringite is the source of expansive power in cement hydration. It is the expanded admixture that makes the most of this crystal growth of ettringite eventually formed in the cement hydration process. Such uses are almost all the hitherto known uses of ettringite, and it may be said that ettringite is known only for these uses.
Recently there has been a tendency to synthesize ettringite in liquid phase, isolate the same and search for possible uses thereof as a material for industry, in addition to its use as an expanded admixture for cement as mentioned above. According to one typical research report by Kondo et al., found in Abstracts of Papers, 1973 Annual Meeting of Ceramic Society of Japan, page 106 and in Seramikkusu (Ceramics, a monthly journal), volume 8, No. 10, pages 67 to 73 (1973), needlelike crystals of ettringite were synthesized from Ca(OH).sub.2, Al.sub.2 (SO.sub.4).sub.3.18H.sub.2 O, CaSO.sub.4, Al.sub.2 O.sub.3 and boehmite (.alpha.-Al.sub.2 O.sub.3.H.sub.2 O) through C.sub.3 A. The crystal of ettringite was 2 .mu.m in length. When the crystals were pressure molded under 500 to 600 Kg/cm.sup.2, the product was not more than 0.07 in percentage of void and not less than 800 Kg/cm.sup.2 in compressive strength, and it was compacted into a translucent, hardened body after being pressed for a long period or dried gradually. The procedures may be regarded as sintering and hot pressing at an ordinary temperature. However, in the case of monosulfate and satin white (granular crystal having a size of 0.1 .mu.m, said to have a chemical composition the same as ettringite, and used as a coating material of paper), such a high strength hardened body cannot be obtained. According to another report by Sugi et al. in Technical Reports of Osaka Cement Co., Ltd., No. 40, pages 23 to 28 (1977), experiments were carried out using finely pulverized blast furnace granulated slag (chemical analysis is 36.8% SiO.sub.2, 19.1% Al.sub.2 O.sub.3, 41.3% CaO) in the synthesis of ettringite using gypsum (gypsum dihydrate) obtained from exhaust gas desulfurization in a slag/gypsum ratio of from 1 to 1.5 with so-called accelerators such as a 10 to 15% aqueous solution of portland cement, Ca(OH).sub.2, KOH or NaOH in an amount about ten times the quantity of the starting materials. After the mixture was stirred for 20 to 24 hours, ettringite was obtained having a purity of about 60%. The ettringites were needle-like or prismatic crystals and 20 to 30 .mu.m in length and 2 to 5 .mu.m in diameter. (refer to FIG. 2).
Although the above reports show that synthetic ettringite crystals have a length of 2 to 30 .mu.m and, when long and within that range, they are prismatic with a relatively large diameter, synthetic ettringite itself, for reasons relative to their crystalline form, aspect ratio and so forth, so far has had no practical use.
Three of the present inventors have already applied for patents on the methods for manufacturing .alpha.-hemihydrate gypsum using an exhaust gas desulfurization process and on the resulting gypsum whisker composites (U.S. patent application Ser. Nos. 882,913 and 890,665), and have disclosed therein a method of producing II-anhydrate gypsum needles, namely gypsum whiskers, useful in industry (from needle crystals of .alpha.-hemihydrate gypsum as a starting material). Due to the fact that the said gypsum whiskers have a large aspect ratio, they are a useful reinforcing material. Because of this, the inventors have tried to prepare highly pure synthetic ettringite whiskers having as aspect ratio much higher than before.